"to mar" means to mess up the appearance of, disfigure. It is used both literally and figuratively. Literally, "the man marred the woman's face when he threw acid on her." Figuratively, "the celebration was marred by violence" meaning violence messed up their celebration. "to ruin" is the physical destruction of. It can be used both literally and figuratively. Literally, "falling bombs ruined the city". Figuratively, "Rain ruined her wedding day." "to destroy" means to completely ruin, put an end to something by damaging or attacking. It can also be used literally or figuratively. Literally, "the boy destroyed the girls doll with a hammer until it was in pieces." Figuratively, "You destroyed my marriage" meaning you caused my marriage to end for some reason such as infidelity. "mar" is less severe as the other two. "marring" just changes the appearance something in a negative way. "ruin" is more severe. "ruining" causes bad damage but not complete damage always. "destroy" is complete damage.